The invention is concerned with ion implanters and particularly with an arrangement for checking the angle of the wafer on a holder to be implanted and the alignment of a travelling Faraday in the implant chamber, used for setting up the ion beam prior to implantation.
Ion implanters have been used for many years in the processing of semiconductor wafers. Typically a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting in the wafer ions of required dopant.
Known ion implanters include batch type implanters and single wafer type implanters. In single wafer type implanters, the ion beam is usually scanned transversely in one orthogonal direction at a relatively high scanning rate, and the single wafer being implanted is mechanically translated to and fro across the scanned beam substantially in a second orthogonal direction. The ion beam can be scanned electrostatically or electromagnetically and it is normal practice to collimate the scanned beam so that the beam impinging on the wafer remains parallel to a desired beam direction during scanning.
The angle at which the ion beam strikes the wafer during an implantation process is important. Many implants are conducted with the ion beam precisely normal to the wafer surface, i.e. where the ion beam has zero angle relative to a normal to the wafer surface. In other processes, implants may be conducted with the normal to the wafer surface at a predetermined angle to the ion beam, for example if it is desired to minimise channelling effects as ions penetrate into the crystal structure of the wafer, or else if it is desired to implant into the walls of trenches or other physical structures on the wafer surface prepared by previous processes, or avoid shadowing by such structures.
It is often important to control the angle of implantation closely, especially for ensuring zero implant angle in certain processes.
It is known in single wafer type implanters for the wafer holder to be adjustable about an axis parallel to the plane of the wafer on the holder, for the purpose of adjusting the implant angle. For example, WO 99/13488 discloses an implant chamber for a single wafer type ion implanter in which the wafer is mounted on a holder and the holder can be translated to and fro through the plane of a scanned ion beam entering the implant chamber. The scanning mechanism can itself be rotated about an axis substantially through the plane of the wafer, so as not only to adjust the angle of the wafer relative to the ion beam, but also to change the direction of the mechanical reciprocating translation of the wafer, so that the wafer holder is always reciprocated in the plane of the wafer.
The above referred International patent application also discloses the provision of a travelling Faraday which can be moved to different positions in the direction of scanning of the beam, for the purpose of checking the rate of dose delivery at different points over the range of scanning of the beam. The travelling Faraday in the above International patent application is also used, in association with a further travelling Faraday upstream in the beam direction, for confirming beam parallelism and scan uniformity.
An object of the present invention is to provide an arrangement in an implanter for accurately confirming the angle relative to the beam of a wafer on the wafer holder, so as to ensure an implant is performed at the correct implant angle. A further object of the present invention is the provision of an arrangement to confirm the alignment of a travelling Faraday used in an implant chamber, so that measurements by the travelling Faraday can be correctly referred to a datum alignment in the implant chamber.
Accordingly, in one aspect, the invention provides an ion implanter having an evacuatable implant chamber, a holder inside the chamber for holding a semiconductor wafer at a selected wafer angle during implantation, said holder being adjustable about at least one axis parallel to the plane of a wafer on the holder for changing said wafer angle, a source of focused electromagnetic radiation mounted relative to said implant chamber and arranged to direct said radiation for reflection at a surface of a wafer held on said holder or at a surface of the holder which is parallel or at a known angle to the plane of a wafer when held on said holder, and a sensor mounted relative to said implant chamber to receive radiation from said source reflected specularly by the wafer surface or by said holder surface, said sensor providing a signal, in response to the reflected radiation, which is indicative of a predetermined wafer angle. The arrangement disclosed above enables the angle of the wafer to be set to the predetermined angle by monitoring the signal from the sensor.
Preferably, said source of electromagnetic radiation and said sensor are mounted outside the implant chamber and the chamber has at least one window transparent to said radiation through which the focused radiation from said radiation source is directed into the chamber, and the reflected radiation leaves the chamber for reception by said sensor. Then, the presence of additional sensing equipment within the implant chamber is avoided, which is important to minimise contamination during implant processes, and also for serviceability of the angle sensing equipment.
In preferred embodiments, the radiation source is arranged to produce an illuminated spot of said reflected radiation on said sensor. Typically the source is a laser arranged to provide a substantially parallel beam of said radiation.
The sensor may have a sensitive area providing a signal representative of the position of the illuminated spot on said area at least in a direction perpendicular to the axis of adjustability of the wafer holder. In this way, the sensor can provide an output signal having a range of values for comparison by control equipment with a desired datum value corresponding to the desired predetermined angle of the wafer.
In a preferred embodiment, the implanter includes a drive unit to adjust the rotary position of the holder about said axis, and a controller responsive to the signal from said sensor to control said drive unit to adjust said holder position to bring said illuminated spot to a desired position on the sensitive area of the sensor corresponding to the predetermined wafer angle.
Normally, the implanter has a source of ions to be implanted, and a scanner and collimator combination to produce a parallel scanned beam of said ions in said implant chamber for implantation in a wafer on said holder. Then preferably, said scanner and collimator combination define an ideal beam direction for said parallel scanned beam, and said radiation source and said sensor are mounted so as to be referenced to said ideal beam direction defined by the scanner and collimator combination, whereby said predetermined wafer angle is referenced to said ideal beam direction.
In one embodiment, said scanner and collimator combination scans the beam in a scan direction perpendicular to the beam direction in a beam scanning plane, and the implanter further includes an actuator for translating the wafer holder through the beam scanning plane in a translation plane containing said scan direction and having a translation angle to the normal to the beam scanning plane, a first rotary drive unit to adjust said translation angle and effecting a corresponding adjustment of said wafer angle, a second rotary drive unit to change the angle of the wafer holder relative to the actuator about an axis parallel to said scan direction to adjust the angle of a wafer on the holder relative to said translation angle, and a controller responsive to the signal from the said sensor to control said first rotary drive unit to adjust said translation angle and said wafer angle to bring said illuminated spot to a desired position on the sensitive area of the sensor corresponding to the predetermined wafer angle, and to control said second rotary drive to adjust the wafer angle relative to the translation angle to minimise any movement of said spot on said sensor when said actuator translates the wafer to move the point of reflection of said radiation over the surface of the wafer.
The present invention also provides an ion implanter having an evacuatable implant chamber, a source of ions to be implanted, a scanner and collimator combination to produce a parallel scanned beam of said ions in said implant chamber for implantation in a semiconductor wafer, a travelling beam detector movable in said implant chamber along a line parallel to the direction of scanning of said beam and usable for checking alignment of the scanned beam, a point source of electromagnetic radiation mounted relative to said implant chamber, and an in-line sensor mounted relative to said implant chamber responsive to the radiation from said source received along a line of transmission crossing said line of movement of the travelling beam detector, the travelling beam detector having a structure which interrupts at least partially said radiation received by the sensor, whereby said in-line sensor provides a signal indicative of at least one predetermined position of the travelling beam detector. With this arrangement, at least one datum position of the travelling beam detector (which may be a Faraday) in its line of travel in the beam scanning direction can be accurately confirmed, so that subsequent use of the travelling beam detector for checking beam alignment can be referenced to the known datum position.
Preferably, the above arrangement for checking the datum of the travelling beam detector can be used in combination with the previously described arrangement for confirming the wafer angle. Then the above referred point source of electromagnetic radiation may be constituted by the source of focused radiation which is also used for checking the wafer angle, and the implanter should then further include an actuator for translating the wafer holder transversely of the plane of the scanned beam. In this way, the wafer holder can be located to intercept the electromagnetic radiation from the source for reflecting this radiation for use in confirming the wafer angle and can also be translated so as to be clear of the radiation from the source so this can be transmitted along the above mentioned line of transmission for use in confirming the position of the travelling beam detector.
In a preferred embodiment, said scanner and collimator combination define an ideal centre line for said parallel scanned ion beam in the implant chamber and said radiation point source directs a beam of said radiation along said line of transmission and is mounted so as to be referenced to said ideal beam centre line so that said line of transmission and therefore said predetermined position of the travelling beam detector are referenced to said ideal beam centre line. Preferably, then, the implanter includes a beam stop to receive the scanned beam, said beam stop including at least one fixed beam detector providing a timing signal indicating the timing of the ion beam scanning over the fixed beam detector at the beam stop, wherein said point source of radiation provides an illuminated spot of radiation on said in-line sensor and said in-line sensor is mounted so as to be referenced to said beam stop and to the position of said fixed beam detector, said in-line sensor having a sensitive area providing a signal representing the position of said illuminated spot on said area at least in a direction parallel to said beam scanning direction, whereby the position of said fixed beam detector relative to the line of transmission can be determined.
Preferably said line of transmission is substantially perpendicular to said line of movement of the travelling beam detector. Said line of transmission may be in a plane containing said ideal centre line and perpendicular to said line of movement.
Preferably, said structure of the travelling beam stop is a flag, and said travelling beam sensor has a slit opening aligned perpendicular to said beam scanning direction for receiving beam ions as the beam is scanned over the detector, and said flag is in a plane perpendicular to the beam direction which contains said slit opening.
Preferably, said in-line sensor is also mounted outside the implant chamber and the chamber has a further window through which said radiation can leave the chamber along said line of transmission.
The invention also provides an ion implanter having an evacuatable implant chamber, a source of ions to be implanted, a scanner and collimator combination to produce a parallel scanned beam of said ions in said implant chamber for implantation in a semiconductor wafer, said scanner and collimator combination defining an ideal centre line for said parallel scanned ion beam in the implant chamber, a source of electromagnetic radiation which directs a beam of said radiation along a line of transmission, and which is mounted so as to be referenced to said ideal beam centre line so that said line of transmission is referenced to said ideal beam centre line, a beam stop to receive the scanned beam, said beam stop including at least one fixed beam detector providing a timing signal indicating the timing of the ion beam scanning over the fixed beam detector at the beam stop, and an in-line sensor mounted on said line of transmission and so as to be referenced to said beam stop and to the position of said fixed beam detector, said radiation beam source providing an illuminated spot of radiation on said in-line sensor, said in-line sensor having a sensitive area providing a signal representing the position of said illuminated spot on said area at least in a direction parallel to said beam scanning direction, whereby the position of said fixed beam detector can be determined relative to said line of transmission and thereby relative to said ideal centre line.
There follows by way of example only a description of preferred embodiments of the invention.